First, Randy is right. The tumbler rating is the total of part plus
media plus filler. The tumbler doesn't have to be completely filled
with abrasive, the best action occurrs when the abrasive powder is
pushed against the parts by another material. I do alot of rock and
metal polishing work in a 12 lb rotary tumber, and if I filled the
entire thing with the silicon carbide grit I use it would cost me $30
per grit size, per load. Instead I use about 1 lb of grit, 5 lbs of
rock or metal, and 5 lbs of gravel or landscaping stone. For the finer
grits I add 5 lbs of plastic pellets that have no abrasive but serve to
cushion the material from getting scratched by other rocks or parts.
The silicon carbide grit is a really good abrasive, I buy 120, 200,
350, and 600 grit 5 lbs at a time. About $5/lb. With a filler to push
the abrasive against the parts, it seems to work great on both metal
and polishing stones.